This invention relates in general to antifriction bearings and, more particularly, to such bearings having superior finishes on their critical working surfaces and a process for providing such bearings.
From the standpoint of antifriction bearings, the tapered roller bearing has a rather complex geometry. A single row tapered roller bearing, which is the most fundamental of that type of bearing, has two races, called the cup and the cone, each provided with a tapered raceway, and tapered rollers arranged in a single row between the raceways of the cup and the cone. Generally speaking, line contact exists between the tapered side faces of the rollers and the raceways. In addition, one of the races, usually the cone, has a thrust rib against which the large ends of the rollers bear. Indeed, were it not for the thrust rib, the rollers would be expelled from the annular space between the raceways, since radial loads transmitted through the rollers translate into small axial components which urge the rollers up the tapers of the raceways--and against the thrust rib.
Initial impressions would suggest that the two raceways and the side faces of the rollers should lie in conical envelopes having their apexes at a common point along the axis of rotation, for this would produce pure rolling contact between the side faces of the rollers and the raceways. In actual practice, the roller side faces and the raceways have slightly curved profiles to alleviate edge loading and accommodate eccentricity imparted by the load. Considerable research has focused on deriving the ideal configurations for raceways and roller side faces, and algorithms exist for determining those configurations. See U.S. Pat. No. 4,877,340.
Apart from that, one would, upon initial consideration, conclude that the large end faces of the rollers should be squared off with respect to the axes of the rollers. Actually the end faces have a slightly spherical profile, for this enables the rollers, upon bearing against the thrust rib, to acquire the proper orientation along the raceways.
The typical bearing is finished to within close tolerances with a final grind on its critical working surfaces. But in some bearings, the finishing along the working surface is carried farther to reduce the run-in period for the bearing and to enable it to operate at lower torques and with less heat generation. These bearings have enhanced finishes. A bearing with an enhanced finish receives rough grind on its critical surfaces, then a finish grind, and finally honing or superfinishing. An enhanced finish possesses some roughness--as do all surfaces--but the roughness is of a microscopic character. Generally, it ranges between 3 and 8 uin Ra (Ra stands for arithmetic mean roughness--uin for microinch or one-millionth of an inch), which is superior to a minimum of 6 to 10 uin Ra, and more often 15 to 25 uin Ra, one finds on the working surfaces of bearings which are finished with nothing more than a final grind. Grinding and honing, however, produce directional surface textures, with the surface irregularities, minute as they may be, extending in the circumferential direction. Indeed, current machining practices leave the circumferential orientation as the only orientation available. But the circumferential orientation does the least to facilitate lubrication along the line of contact between the raceways and the side faces of the rollers.
In this regard, N. Patir and H. S. Cheng, in their paper entitled An Average Flow Model for Determining Effects of Three-Dimensional Roughness .oOn Partial Hydrodynamic Lubrication, Transactions of the ASME, Vol. 100, page 12, January 1978, examined orientations of microscopic surface irregularities in connection with their effects on lubrication and concluded that a surface composed of irregularities extended longitudinally in the direction of movement renders the lubricant less effective than a surface having irregularities extended transversely to the direction of movement, and that an isotropic surface, that is one which has no orientation to its irregularities, is superior to a surface composed of longitudinal orientations. Translated to bearings, the typical machining process leaves the raceways and roller side faces with machining marks in the form of circumferentially directed grooves which tend to diminish the thickness of the hydrodynamic oil film which exists between the raceways and rollers, and this holds true even with respect to bearings having enhanced finishes.
So-called barrel finishing, that is to say, tumbling a part within a corrosive environment, will leave a surface with minute irregularities that are quite shallow, but the traditional barrel finishing procedures destroy the precision profiles to which the roller side faces and raceways are machined. Thus, traditional barrel finishing does not represent an acceptable substitute for producing a bearing with an improved finish.
The present invention resides in a process for providing superior working surfaces for bearing components. Usually the races and rollers on which those working surfaces exist are machined, ultimately by grinding, such that those surfaces have the desired profiles. Then some or all of these components are subjected to a further finishing which leaves the surface with irregularities on the order of 1 to 3 uin Ra. The irregularities have no orientation and thus render the surface isotropic. The invention also resides in an antifriction bearing having working surfaces with isotropic finishes.